The present invention relates to a sample position controller in a focused ion beam system, and particularly relates to a sample position controller of a focused ion beam system adapted to failure analysis of a material such as a semiconductor memory element in which a pattern repeated at regular intervals exists in its surface.
The conventional focused ion beam system is generally known as a system for finely machining various kinds of materials. The system has been recently raised in estimation of the usefulness thereof as a system for analyzing a defective bit of a semiconductor memory element.
In the application of the focused ion beam system to defective bit analysis of a semiconductor memory element, firstly, the position of a failed cell in the memory element in which a failure occurs is calculated in the form of position data (address) in a bit unit which is a memory unit while all memory cells are operated actually. After the position of a defective bit in the defective memory element is determined as position data, the defective memory element is set on a movable sample stage of the focused ion beam system. Then, an operation of moving the sample stage for position control is carried out so that the defective bit portion of the defective memory element is made to come to a focused ion beam working position which is set in the focused ion beam system. After the defective bit portion is set to the focused ion beam working position, a position slightly in front of the position of a cross section to be formed in the defective bit portion is etched largely by an ion beam. Then, the side wall of the etched position is etched finely gradually to form a cross section in the target position. Finally, a cross section is formed accurately in the position of the defective bit portion. Then, this cross section is observed by using an electron microscope or the like to diagnose the failure as to the structure and kind thereof to thereby perform analysis for making an improvement in the process of producing a semiconductor memory device. Such analysis is briefly described in IEEE/IRPS, 1989, pp. 43-51.
In the aforementioned application of the focused ion beam system to defective bit analysis of a memory element, it is essential to the operation of the system that the defective bit portion is accurately moved to the focused ion beam working position. In the conventional focused ion beam system, the moving operation for position control is conducted manually by a human operator. That is, a human operator observes the sample by using the focused ion beam system to count both the number of columns of memory cells and the number of lines of memory cells to thereby obtain the target defective bit position. In this method, however, not only the counting is liable to error while the numbers are counted up to the order of hundreds or thousands but a large time is required for counting the numbers.
A proposal to improve the sample stage moving operation is disclosed in JP-A-63-312627. This proposal is however the same as the aforementioned method in that the quantity of movement of the sample stage is manually counted as the numbers of columns and lines of memory cells.
As position control techniques for adjusting the position of the sample stage, there are a device working position control system using a focused ion beam as disclosed in JP-A-2-24949 and a sample position display system of a charged particle beam analyzer as disclosed in JP-A-58-75749.
In the application of the conventional focused ion beam system to,defective bit analysis, as described above, the moving operation for position control to obtain the defective bit portion is based on operator's measuring the quantity of movement while counting the number of bit portions on the surface of the memory element through observation by using a microscope or the like. In such a moving method, however, in today with the reduction of the size of bit portions as memory units and the increase of the number of bit portions caused by the enlargement of the semiconductor memory element both in density and in capacity, it is practically very difficult to accurately and rapidly move the electrically determined defective bit portion of the semiconductor memory element to a working position by means of a focused ion beam. This brings a great lowering of operating efficiency.